This invention relates to a near infrared analyzer for quantitative and/or qualitative analysis of a sample by measuring the optical characteristics of its transmitted and/or reflected light in a near infrared region. Analyzers of this kind have been in use for measuring spectroscopic characteristics, for example, of agricultural products, processed foods, chemical substances and medicines against light.
Near infrared analyzers were developed initially as an apparatus for analyzing the protein and water content of wheat and have since come to be used extensively for the analysis of processed foods, liquors, products of chemical industries and medicines. When a near infrared analyzer is used for analysis, the sample is generally not pretreated chemically, that is, the sample is usually used in the apparatus in a non-destructed condition. According to an exemplary method of quantitative analysis, an absorption or reflection spectrum of an unknown sample is measured over a certain range of wavelength principally in the near infrared region or at a plurality of near infrared wavelengths, and the content of a desired component is obtained by using the spectral data with a preliminarily prepared predictive formula. For a simultaneous quantitative analysis of a plurality of components, predictive formulas are individually prepared for the desired components and individually applied to the spectral data. Such predictive formulas are prepared by a statistical method from the spectral data of a plurality of samples with a variety of known contents. Where a quantitative analysis is the objective, such formulas may be prepared, say, by a multiple regression analysis. These predictive formulas are similar to what is commonly known as the detection line in ordinary instrumental analyses but involve much more complicated calculations.
Since such analyzers are used principally for sorting agricultural products and quality control of processed foods, they mostly perform analyses of similar kinds many times day in and day out. Since the results of these measurements are very important in the quality control and determination of the prices of the measured samples, the analyzers must be able to perform the measurements speedily, their accuracy must be at least as good as by prior art methods of chemical analyses, and stability must be high both on a long-term basis and on a short-term basis.
From the point of view of the structure of spectroscopic elements to be used, prior art analyzers which have been used to satisfy these requirements can be classified into the kind which scans a wavelength band of the detection light and the kind which performs measurements at a fixed wavelength. The latter is characterized by the a plurality of band pass filters within a narrow band which are sequentially inserted into an optical path such that absorptivity can be measured at a plurality of wavelengths. Analyzers of the latter (fixed wavelength) kind are advantageous in that the structure is simple and they can be inexpensive. They are disadvantageous, however, in that the wavelength cannot be freely selected for measurements and hence that the kind of samples which can be measured becomes limited, depending on the analyzer to be used.
Analyzers of the former (wavelength-scanning) kind may be characterized as having a diffraction grating and scanning a wavelength band of light emitted through an exit slit by rotating or oscillating the grating. The wavelength of the light to be used for analysis can be freely selected with an analyzer of this kind, and such an analyzer can be used for a larger variety of samples and for broader research purposes. In order to satisfy the requirements mentioned above with an analyzer of this kind with a diffraction grating, that is, to achieve high-speed measurements with high accuracy with high short-term and long-term stability, however, many improvements must be effected. For example, the optical system inclusive of a spectrometer must be designed for low optical losses, both the speed and accuracy of the scanning of the diffraction grating must be improved, and the temperature variations and vibrations of each component of the device must be reduced or compensated for. In view of the above, U.S. Pat. No. 4,283,596 disclosed an optical system using a unique cam drive structure to oscillate a diffraction grating. U.S. Pat. No. 4,540,282 disclosed a combination of a return force spring, a detector of back electromagnetic flux and a DC motor for providing a harmonic oscillation to a diffraction grating. European Patent Application Publication No. 378,108 disclosed a combination of a DC motor and an optical encoder to control the speed of the motor to oscillate a diffraction grating. In addition, it has been known, as has been used in visible-light, ultraviolet and infrared spectrophotometers, to convert the rotary motion of a motor into a linear motion by means of a feed screw and to convert it again for the rotation of a diffraction grating by means of a sine bar.
There have been problems with prior art near infrared analyzers, and in particular with those which use a diffraction grating spectrometer to scan a wavelength band by rotating or oscillating the diffraction grating. If the grating is mechanically scanned, as with all of the devices described above, a complicated power-transmitting mechanism such as a cam is necessary and, since the speed of its rotation must be increased in order to reduce the time of measurement, long-term maintenance of mechanical reliability and accuracy becomes a serious problem. In the case of devices with a combination of a DC motor and an encoder, it must be remembered that components like the encoder requiring high accuracy are extremely vulnerable to vibrations and shocks. In the case of measurements of agricultural products outdoors where there are noises and severe temperature changes, for example, there arise short-term problems of reliability. Moreover, both the motor and the encoder are rotary machine components, but bearings have a limited mechanical lifetime. One cannot forget the additional problems of deterioration in accuracy of measurements due to wears and tears of parts over a long time of use.
It is therefore an object of the present invention to provide a near infrared analyzer capable of high-speed measurements with high accuracy which is stable both on long-term and short-term bases.